Southernly from Alonissos, in distance of 5,8 naut. miles fromPatitiri are located the rocky islands with the name Dio Aderfia (means Two Brothers) that are separated from each other from a narrow band of sea.

The island of Piperi and the sea surrounding it form the kernel of the National Marine Park. At the same time it is the most inaccessible island as it has no protected bay for the approach of a caïque. Steep rocks descend vertically into the sea on all sides. This rocky island has an area of 7 square kilometres and its high points are between 274 and 352 metres. There are clumps of pine and isolated oaks.